Abstract: Based on analyzing failure cases due to hairline crack defects, the common patterns, characteristics and harms of hairline crack defects in engineering were discussed. It is found that hairline crack defects present diverse patterns, including one-dimension linear pattern, two-dimension flake pattern, and three-dimension small angle section pattern. There are both long hair cracks distributed intermittently or continuously in the same direction and short ones distributed at random. In addition, hairline crack defects present various micro morphology and have different contents within them. Hairline cracks have many harms, such as promoting initiation of cracks, leading to fake crack presentation during nondestructive detection, resulting in unacceptable surface defects, and leading to leakage failure of leak-proof parts. Based on summarizing their patterns and harms, the factors contributing to hairline crack defects can be more easily determined, and then proper prevention measures and evaluation methods can be put forward, so that necessary demands for structure design, material and examination can be come up with in the early stage of structure design to improve the quality and reliability of products.